Tuning Green Upconversion in CaY2Sc2−xMxAl2GeO12:Er3+ Garnets (M = Al, Ga, In) Through Octahedral Cation Substitution

Rare-earth-doped upconversion phosphors continue to attract interest for optoelectronics and bio-labeling, yet their low conversion efficiency remains a persistent bottleneck. Here we report how substitution of the octahedral-site Sc3+ in CaY2Sc2Al2GeO12 (CYSAG) garnets by Al3+, Ga3+ or In3+ modulates the Er3+ upconversion behavior. A series of Er3+-doped garnets was prepared by solid-state reaction, and all samples were confirmed to be single-phase cubic garnets. Under 980 nm excitation, the Ga3+-substituted phosphor exhibits the strongest green emission, reaching 2.6 times that of the Sc-only reference, together with the longest 4S3/2 lifetime (~47 μs). The enhancement is rationalized in terms of an optimized local crystal field around Er3+ and suppressed multiphonon relaxation. The results identify octahedral-site cation engineering as a simple and effective lever for tuning upconversion efficiency in garnet hosts.